For some years, the concept of a dual-voltage electrical power supply system has been developed in order to respond to the increase in the number and the electrical consumption of the devices fitted on board motor vehicles.
Patent application FR 2836604 describes this type of dual voltage power supply system.
One part of the system is powered by a low-voltage DC energy storage element, generally consisting of a 12V battery.
Another part of the system is powered by a high-voltage electrical energy storage element, generally consisting of an ultracapacitor charged at variable voltage, via a regulator, by an alternator driven by the thermal engine of the vehicle.
The low voltage system is used chiefly to power low electrical loads, such as dashboard or cabin lighting, the radio, door lock actuators, indicator lights, etc.
The high voltage system serves to power the on-board equipment requiring high electrical power, such as the starter of the thermal engine, the headlamps, or the air conditioning system.
Transfers of electrical energy between the two parts of the system are effected by means of a reversible DC-DC converter, and are governed depending on the resources in demand and available on either of the systems, in particular as indicated in the aforementioned patent application.
The voltage of the low voltage system being substantially constant, the items of equipment connected in parallel each have only one operating point linking voltage at the terminals and current, for a given electrical power.
In the case of a classic on-board electrical system, with set voltage, powered by an electrical energy storage element associated with an alternator regulated to this fixed voltage, the only possible way to optimise the efficiency of the system appears to be to manage the utilisation of either the alternator, or the battery, or a combination of the two as depending on the energy demand, as proposed by the patent application US 20020171392.
On the other hand, since the voltage of the high voltage system is variable, the devices connected in parallel on this system, whether they are consumers or generators of electrical energy, present several possible operating points for equal power, drawn or generated.
This supplementary degree of liberty, offered by an architecture for a variable voltage electrical power supply system, seems until now not to have been taken into consideration by the person skilled in the art in order to provide optimal management of the energy available on board a motor vehicle.